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Ca II HK Stellar Cycle Amplitudes Vs. Rossby Number

Presentation #131.01 in the session “Stellar Cycles, Convection, & Flares”.

Published onJun 18, 2021
Ca II HK Stellar Cycle Amplitudes Vs. Rossby Number

As shown by the groundbreaking Mt. Wilson Observatory HK Project, nearly half of cool stars (those with outer convective envelopes) exhibit solar-like stellar cycles, driven by magnetic activity and revealed by their chromospheric CaII HK emission. Photometric monitoring has also discovered hundreds more stars with cycles, but photometric cycle amplitudes are much smaller and harder to interpret than in the HK lines, since intensity variations can be synchronized with magnetic activity (as in the Sun) or anti-correlated, or even undetectable, depending on stellar age, type, and general activity level. In contrast, chromospheric HK and coronal X-ray emission is always in sync with magnetic activity. Past work has shown that average HK and X-ray emission levels are correlated with rotation period and Rossby number Ro (with a break around Ro=0.2), but little work has been done on cycle amplitudes. From the handful of stars with X-ray monitoring, cycle amplitudes appear to be correlated with Rossby number, with a break at higher Ro values (somewhat less than 1). We investigate whether CaII HK intensity follows the same pattern, extracting cycle amplitudes for dozens of stars from papers published over the past ~25 years.

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